This invention relates to grips which are used to hold a compact fracture specimen while it is loaded in tension and compression by a hydraulic load frame.
Initiation and controlled propagation of cracks must be produced in various types of materials such as beryllium, ceramics, and various steels under tension/compression cycling in order for subsequent fracture toughness or crack growth rate testing to be valid. Such testing is described in S-200 Grade Beryllium Fracture Properties, Journal of Testing and Evaluation, JTEVA, Vol. 1, No. 2, March 1973, pp. 110-118.
The above described testing may be carried out with, for instance, an 810 Material Test System, manufactured by MTS, Inc., Eaton Prairie, Minn. However, the sample holding grips supplied by MTS are not suitable for holding the specimen to be tested in both tension and compression. The standard grips allow for only tensile loading of the specimen through two tension pins that are of smaller diameter than corresponding holes in the specimen. It is the clearance of the pins that pass through holes in the specimen that precludes cyclic loading of the specimen in both tension and compression.
The use of set screws to apply pressure to the top and bottom edges of the specimen, in order to achieve the compressive part of the loading cycle, are not effective. The set screws are not capable of producing an adequate pressure on the specimen to prevent vibration during the loading cycles. This will lead to frequent loosening of the assembly which, in some cases, will result in the tension pins falling out and the test terminating early. The use of an adjustable pin, as suggested in the Journal of Testing and Evaluation, supra, also proved ineffective. Vibration during testing resulted in a loosening of the tension pins.
Accordingly, it is an object of the present invention to provide grips for tension/compression cycle testing, which hold the specimen tightly in position and do not loosen during the testing cycle.